The present invention relates to an Ni-based or Nixe2x80x94Co-based heat-resistant alloy wire, which has a xcex3 phase (austenite) metal structure, for use mainly as material for springs for various parts that require to have heat-resistant quality, such as engine parts, parts for nuclear power generation, and turbine parts.
As a material for springs used in gas-exhausting systems for engines of automobiles, austenitic stainless steel conventionally used as heat-resistant steel, such as SUS 304, SUS 316, or SUS 631J1, has been used for operating temperatures ranging from normal temperature to 350xc2x0 C. An Ni-based heat-resistant alloy, such as Inconel X750 or Inconel 718 (brand names), has been used as material for parts used in temperatures over 400xc2x0 C.
Recently, there is growing demand for more stringent control of the exhaust gases of automobiles as a measure for environmental protection. The demands have prompted a tendency to raise the temperature of the gas-exhausting systems in order to increase the efficiency of engines and catalysts. As a result, the operating temperature of springs, which thus far, have been usually used at about 600xc2x0 C., has risen to about 650xc2x0 C. In this case, even an Ni-based heat-resistant alloy, such as Inconel X750 or Inconel 718, may be insufficient in heat-resistant quality, especially resistance to sag at high temperatures, the resistance of which is particularly required of heat-resistant springs. In such a case, Nixe2x80x94Co-based heat-resistant alloys, such as Waspaloy and Udimet 700 (brand names), may be taken into consideration as alloys that can be used at the highest temperatures thus far. They do not, however, necessarily have excellent resistance to sag at high temperatures.
The foregoing Ni-based alloy and Nixe2x80x94Co-based alloy are strengthened alloys in which xcex3xe2x80x2 phases (precipitated phases having Ni3A as a fundamental form) are intensively precipitated in the xcex3 phase (austenite phase), which acts as a matrix. The structures in the matrix and xcex3xe2x80x2 phase must be controlled to improve the heat-resistant quality.
The published Japanese Patent Application Tokukoushou 48-7173 limits the amounts and ratios of added elements, such as Mo, W, Al, Ti, Nb, Ta, and V, in order to obtain high-temperature strength at temperatures over 600xc2x0 C.
Another published Japanese Patent. Application, Tokukoushou 54-6968, limits the contents of and added ratios between Mo and W and the contents of and added ratios between Ti and Al in order to obtain high-temperature strength, resistance to corrosion, and resistance to brittle fracture.
However, these inventions focus on improving the heat-resistant quality (mainly high-temperature strength) mainly by controlling the precipitated phase as opposed to improving the resistance to sag at temperatures over 600xc2x0 C., the resistance of which is required of heat-resistant springs. Alloy wires for heat-resistant springs are produced through the steps of melting, casing, rolling, forging, solution heat treatment, wire drawing, spring formation, and aging heat treatment. The formation of a texture in the matrix (xcex3 phase) and the change in crystal-grain diameter during the above process, also significantly affect the heat-resistant quality of the products.
In view of the above circumstances, the main object of the present invention is to offer a heat-resistant alloy wire with excellent resistance to sag at high temperatures ranging from 600 to 700xc2x0 C., which is strongly required of spring materials. The excellent resistance to sag is obtained by controlling the crystal-grain diameter of the xcex3 phase, which is the matrix of an Ni-based or Nixe2x80x94Co-based heat-resistant alloy, and by controlling the precipitation of the xcex3xe2x80x2 phase [Ni3(Al,Ti,Nb,Ta)].
The heat-resistant alloy wire of the present invention has the following features:
(a) It contains 0.01 to 0.40 wt % C, 5.0 to 25.0 wt % Cr, and 0.2 to 8.0 wt % Al.
(b) It contains at least one constituent selected from the group consisting of 1.0 to 18.0 wt % Mo, 0.5 to 15.0 wt % W, 0.5 to 5.0 wt % Nb, 1,0 to 10.0 wt % Ta, 0.1 to 5.0 wt % Ti and 0.001 to 0.05 wt % B.
(c) It contains at least one constituent selected from the group consisting of 3.0 to 20.0 wt % Fe and 1.0 to 30.0 wt % Co.
(d) It has the remainder consisting mainly of Ni and unavoidable impurities.
(e) It has a tensile strength of not less than 1,400 N/mm2 and less than 1,800 N/mm2.
(f) It has an average crystal-grain diameter not less than 5 xcexcm and less than 50 xcexcm in its cross section.
(g) It has a crystal-grain aspect ratio (major-axis/minor-axis ratio) of 1.2 to 10 in a longitudinal section.
The alloy wire of the present invention is mainly used as material for springs. Therefore, after undergoing the wire-drawing process, the wire must be formed into a spring by a coiling process. In consideration of the required tensile strength for the coiling process and the possibility of breakage during the process, the wire is required to have a tensile strength of not less than 1,400 N/mm2 and less than 1,800 N/mm2.
If the crystal-grain aspect ratio is less than 1.2 or more than 10 in a longitudinal section, sufficient resistance to sag at high temperatures cannot be achieved.
In order to further improve the heat-resistant quality, it is desirable that the alloy wire before undergoing the coiling process have an average crystal-grain diameter of not less than 10 xcexcm in its cross section. This lower limit is to decrease the number of grain boundaries so that the total displacement can be reduced when sliding occurs at the grain boundaries. If the average crystal-grain diameter becomes 50 xcexcm or more in a cross section, the tensile strength at room temperature required for the spring formation process cannot be achieved. Hence, the diameter must be less than 50 xcexcm. Here the average crystal-grain diameter in a cross section shows the one in the foregoing xcex3 phase.
In order to control the crystal-grain diameter, it is effective to raise the temperature for the solution heat treatment. Specifically, when the solution heat treatment is carried out at a temperature of not lower than 1,100xc2x0 C. and lower than 1,200xc2x0 C., the specified crystal-grain diameter can be obtained easily in a short time. Even if the solution heat treatment is carried out at a temperature of not lower than 1,000xc2x0 C. and lower than 1,100xc2x0 C., when the wire drawing is performed at a reduction rate in the area of 5% to 60%, desirably 10% to 20%, an alloy wire excellent in resistance to sag at high temperatures can be obtained.
The alloy wire of the present invention is a heat-resistant alloy wire in which xcex3xe2x80x2 precipitation is intensified. The alloy wire treated by the foregoing control of the crystal-grain diameter is formed into a spring. Subsequently, a proper aging heat treatment is selected and carried out at a temperature of not lower than 600xc2x0 C. and lower than 900xc2x0 C. for a period of not less than one hour and less than 24 hours. Thus, the required high heat-resistant quality can be obtained. The xcex3xe2x80x2 phase can be detected through X-ray diffraction.
In the present invention, the selection of the constituent elements and the limitation of the constituting ranges are conducted for the following reasons:
The element C increases the high-temperature strength by combining with Cr and other elements in the alloy to form carbides. However, an excessive amount of C decreases toughness and corrosion resistance. Consequently, 0.01 to 0.40 wt % C is determined as an effective content.
The element Cr is effective to obtain heat-resistant quality and oxidation resistance. First, an Ni equivalent and a Cr equivalent are calculated from the other constituent elements in the alloy wire of the present invention. Then, considering the phase stability of the xcex3 phase (austenite), 5.0 wt % or more Cr is determined to obtain the required heat-resistant quality. In view of the toughness deterioration, 25.0 wt % or less Cr is determined.
The element Al is the principal constituent element of the xcex3xe2x80x2 phase [Ni3(Al,Ti,Nb,Ta)]. It easily forms oxides and is also used as a deoxidizer for melting refinement. An excessive addition of Al, however, easily causes deterioration in hot-working quality. Consequently, 0.2 to 8.0 wt % Al is selected.
The elements Mo and W form a solid solution with the xcex3 phase (austenite) and contribute considerably to the increase in high-temperature tensile strength and resistance to sag. On the other hand, they tend to form TCP phases, such as a "sgr" phase, that decrease creep fracture strength and ductility. In considering the minimum added amount required to improve the resistance to sag and of the deterioration in processibility, 1.0 to 18.0 wt % Mo and 0.5 to 15.0 wt % W are determined.
In the alloy wire of the present invention, xcex3xe2x80x2 phases, namely [Ni3(Al,Ti,Nb,Ta)], are intensively precipitated to improve the heat-resistant quality. The constituting ranges of the constituent elements are limited for the following reasons:
The element Ti is the principal constituent element of the xcex3xe2x80x2 phase [Ni3(Al,Ti,Nb,Ta)]. However, the excessive addition of Ti causes the excessive precipitation of an xcex7 phase (Ni3Ti: an hcp structure) at the grain boundaries. As a result, it is unable to control the precipitation of the xcex3xe2x80x2 phase [Ni3(Al,Ti,Nb,Ta)] required to obtain heat-resistant quality by heat treatment only. In order to secure an effective amount of the precipitation, it is necessary to limit the element to 0.1 to 5.0 wt % Ti.
The element Nb precipitates an Fe2Nb (Laves) phase if excessively added. In order to avoid the resultant strength reduction, 0.5 to 5.0 wt % Nb is determined.
The element Ta is, as with Nb, a ferrite-stabilizing element. Therefore, it deprives the xcex3 phase of its stability if excessively added. In order to avoid excessive precipitation in the grain boundaries, 1.0 to 10.0 wt % Ta is determined.
The element B is added to prevent a hot shortness and increase the toughness in intensively precipitating the xcex3xe2x80x2 phase in order to strengthen the xcex3 phase. For this purpose, 0.001 to 0.05 wt % B is determined.
The elements Co and Fe form a solid solution with Ni and exist in high concentrations in the xcex3 phase. The element Fe is useful for reducing the production cost of alloys. However, it may reduce the amount of precipitation of the xcex3xe2x80x2 phase or form a Laves phase with Nb or Mo. Consequently, 3.0 to 20.0 wt % Fe is determined. The element Co has the following functions:
(a) reducing the stacking-fault energy;
(b) intensifying the solid solution hardening;
(c) raising the temperatures for the solubility limit of the xcex3xe2x80x2 phase in the grain boundaries;
(d) raising the allowable operating temperatures of the alloys;
(e) increasing the amount of precipitation of the xcex3xe2x80x2 phase in the crystal grains; and
(f) suppressing the growth of the grains of the xcex3xe2x80x2 phase (the xcex3xe2x80x2 grains) in the crystal grains.
Consequently, 1.0 to 30.0 wt % Co is determined as an effective content.